Silicon has been studied for a long time as anode material for lithium batteries[1] because of its exceptional ability to accumulate lithium, which results in a very high energy density by weight, which can theoretically reach 3700 mAh/g. Crystalline silicon powder in the form of microparticles is furthermore a relatively inexpensive material which is a byproduct of the CMOS electronics industry. However, lithium batteries based on silicon anodes show a large fall in the electrical storage capacity after a small number of charge/discharge cycles because in particular of the fracturing of the silicon due to the lithiation/delithiation cycles[2] and of the loss of electrical connection with a major portion of the silicon of the anode[3].
In order to limit the loss of charge, the silicon can be introduced in the form of nanoparticles[1]. However, the electrical contact of the nanoparticles with the electrode has to be ensured by the addition of a conductive component, such as carbon black. During the cycles, a portion of the nanoparticles losses contact with the electrode[3].
The use of silicon nanowires[4] makes it possible to retain a high energy density by weight, to prevent fracturing of the silicon and to retain a good electrical contact of the silicon with the electrode by virtue of the elongated shape of the nanowires, which promotes network contacts.
In supercapacitors, the electrodes comprising silicon nanowires also represent an advantageous alternative to carbon as they show a high specific surface[5], a high conductivity if the nanowires are (ultra)doped[6], and a high chemical stability to cycling even with high voltages[7], which ensures a high energy density stored in the supercapacitor.
Of course, the silicon nanowires have to be specially synthesized for the application, in contrast to silicon nanopowders recovered as cheap industrial byproduct. It is thus important for any industrial application of this material to develop a synthesis appropriate for the large scale and having a low cost.
Several strategies have been developed for the synthesis of silicon nanowires:
1/ micromanufacture by etching of a bulk silicon block[8],
2/ synthesis of thin layer type by CVD (Chemical Vapor Deposition)[9],
3/ volume-based chemical synthesis in the liquid phase[10] or supercritical phase[11],
4/ vapor phase chemical synthesis[12] over substrate,
5/ gas phase pyrolytic synthesis[13] over substrate.
The majority of the methods described (cases 1/, 2/, 4/ and 5/) produce silicon nanowires as a thin film at the surface of the substrate, the weight of the nanowires produced being very low (a few μg at the best). The production yield, understood as the weight of silicon in the form of nanowires with respect to the weight of silicon used in the process, is very low (<<1%); the production cost is thus very high.
The volume-based chemical liquid-phase synthesis[10] and in the supercritical phase[11] (case 3/) provided by the team of Pr B. A. Korgel make it possible to obtain a larger amount of silicon nanowires. These methods have also been described in the international application WO 2011/156019. The yield is high (60%) in the case of the supercritical-phase synthesis alone; it is low in the liquid-phase synthesis (<5%). However, the liquid-phase synthesis uses a highly pyrophoric reactant, requiring handling in a glovebox; the supercritical-phase synthesis is based on an expensive process under high pressure which is difficult to adapt industrially. Furthermore, these methods have not made it possible to date to dope the nanowires during the synthesis.
Finally, the pyrolytic synthesis provided by Pr S. B. Rananavare[13] makes possible a bulk synthesis using, as synthesis substrate, a sacrificial porous material, chalk or glass wool. The silicon nanowires obtained according to this synthesis have diameters of the order of 20 to 100 nm. In this type of synthesis, the diameter of the nanowires is determined by the diameter of the catalyst nanoparticles. Although the gold nanoparticles used in this study have small diameters of 5 to 10 nm, they diffuse under the effect of the heat to the surface of the support and amalgamate, which results in the formation of nanoparticles with greater diameters and thus in the growth of nanowires with a diameter of 20 to 100 nm.